It is well known that in order to improve fuel efficiency and emissions it is desirable to produce a lightweight motor vehicle. In order to achieve this goal, several vehicle manufacturers have proposed vehicles in which a substantial proportion of the body structure or bodyshell is constructed from a lightweight material such as an aluminum alloy. An example of such a vehicle construction is shown in U.S. Pat. No. 6,099,071.
Some sections of such a bodyshell act as beams for which a substantial yield strength is a principal requirement. For a bodyshell assembled from aluminum pressings and extrusions, the yield strength of the aluminum dictates the thickness of material required in these places. This can result in the design being bulky, heavy and expensive compared to a design using high strength steel, thereby eliminating much of the anticipated weight saving from the use of aluminum.
One way of optimizing the use of aluminum is to integrate a steel reinforcement into the structure. However, many existing methods of joining steel and aluminum such as brazing, adhesive bonding, bolting, or riveting have disadvantages in motor vehicle production, particularly for mass-produced passenger cars. In particular, brazing provides inadequate joint strength. Adhesive bonding requires supplementary mechanical fixings such as riveting or bolting to avoid peel failure and to fix the geometry of an assembly prior to curing. Bolting is time consuming and difficult to accommodate where space on a component is limited, e.g., in the posts, or pillars of the upper structure of a bodyshell. Blind riveting is more space efficient than bolting but can be even more time consuming.
One quick, space efficient and repeatable method for fastening together bodyshell components in a mass production environment is the use of a self-piercing riveting process such as described in U.S. Pat. No. 5,752,305, hereby incorporated by reference.
Hence it is desirable to be able to join a high yield strength steel reinforcement to an aluminum structure using self-piercing rivets since this can be done in a cost effective manner and produces a joint with the required strength. For steels having a low to moderate yield strength (e.g. up to 440 MPa) such joining is possible by directly fastening the steel to aluminum using self pierce riveting. However, such steels do not have a sufficiently high yield strength to produce the required combination of low weight and resistance to bending. When steel having a sufficiently high yield strength (e.g. 950 MPa) is used, then self-piercing rivets cannot be used because the steel is too hard to yield during the riveting process.
The reinforcement of aluminum profiles with steel is known, e.g., from BE511181 where an aluminum glazing profile is shown and from JP57-001516A where a trolley wire is described as being made by extruding an aluminum coat over a steel core. However, in neither case is the profile suitable for a structural member of a motor vehicle where high strength has to be combined with the ability to readily join the structural member to other components of the bodyshell. While in U.S. Pat. No. 5,941,597, a steel reinforcement of an aluminum extrusion has been proposed for such a structural member, this is simply fitted inside the extruded section to increase stiffness.
It is thus an object of this invention to provide a structural member for a motor vehicle bodyshell which includes a reinforcing member of low weight and a high resistance to bending and can be easily joined to other components of the bodyshell.